The Wittig rearrangement as a practical method for aldehyde synthesis

If the rearrangement of metalated allyl ethers, e.g., I, is accomplished in the presence of potassium tert-butoxide, primary alkyl groups preferentially migrate to the unsubstituted allylic terminus (g-position). Enolates (Z)-R3CHR4CR1R2CH2CH:CHOLi and 1-vinylalcoholeates (Z)-CH2:CHCH(OLi)CR1R2CHR3R4 (by alkyl migration to the a-position, adjacent to the oxygen atom) are produced in an approx. ratio of 9:1. Because of the endo-configuration of their organometallic precursors, the enolates exclusively emerge in the (Z)-configuration as shown by trapping with chlorotrimethylsilane and isolation of the resulting O-silyl (Z)-enethers. Hydrolysis of the latter affords the corresponding aldehydes with good yields. The rearrangement is mechanistically still obscure. A concerted process as the main reaction mode is unlikely. The intermediacy of zwitterionic metallomers, e.g., II, and solvent caged radical pairs, e.g., III, is tentatively suggested. [on SciFinder (R)]